1. Field of the Invention
This invention relates generally to high-speed serial-type wire printers, and more particularly to media thickness compensation means for insuring maintenance of the print head a predetermined distance from the impression-receiving media surface regardless of the thickness of the media.
2. Description of the Prior Art
Some high-speed serial-type wire printers are designed to accommodate different thicknesses of impression-receiving media either at different times or simultaneously, such as accounting machines for printing on a tally sheet and, as a part of the same operation, on a pass book. To accommodate such different thicknesses of media, the print head has been mounted on its carriage, which is movable transversely of the platen along a print line, for additional movement relative to the carriage perpendicular to the print line, and provided with spring means urging the print head toward the platen and sensing means for engaging the print or impression-receiving surfaces of the media that are backed by the platen. In one such machine, a roller is mounted on each side of the rear or printing end of the print head, but this results in inaccurate positioning of the printing ends of the wires for the lateral distance between the wires and the center of the roller on the advancing side of the print head and also causes lateral torque or bending forces being imposed on the print head whenever only one of the rollers is in engagement with the media.
Another prior art machine is that disclosed in U.S. Pat. No. 3,750,792 which provides a flexible tape spring-urged into contact with the impression-receiving media that is backed by the platen, and a sensing finger on the rear or printing end of the print head which engages the tape. This results in an artificial gradual movement of the print head perpendicular to the print line rather than the actual abrupt transition between media of different thicknesses, due to the nature of the tape. Both of these prior art machines also are unsatisfactory because in being spring-urged into engagement with the impression-receiving surfaces of the media, the sensing rollers and finger cause streaking of the lower sheets of multiple sheet forms having carbon or other pressure-responsive reproducing means incorporated therein.